I have an older Honda 10 hp outboard that has a charging circuit. The
small boat that it is on has a bilge pump and small AGM battery. The
outboard's charging circuit puts out over 15 volts. Can I put a diode
in the circuit to reduce the voltage? If so what kind of diode?

thanks

I agree on the zener diode because it is the only diode type that not
only forces the direction of the current (normal diode) but limits
voltage. Flooded battery better but since you already own the AGM, you
could put some type of RESISTIVE material in series. (Large resistor,
light, etc) The key would be to find the right resistance (Approx
2/15th of the internal resistance of the battery) But experimentation
would be reguired.



wrote:
On Mon, 12 Nov 2007 12:12:16 -0800, wrote:


I have an older Honda 10 hp outboard that has a charging circuit. The
small boat that it is on has a bilge pump and small AGM battery. The
outboard's charging circuit puts out over 15 volts. Can I put a diode
in the circuit to reduce the voltage? If so what kind of diode?

thanks


A very large zener diode with a substantial heat sink. I doubt you will find one
that can handle the current. Old British motorcycles had them for this purpose,
but that was a LONG time ago, and they were more likely 6 volt systems.

Most small outboards have poor, or no, regulation to speak of. Some
manufacturers have add on regulators, but they are pretty expensive - as in
$200+. It would make more sense in your case, to ditch the AGM and get a flooded
battery that is not so sensitive to voltages over 14. You will need to check the
electrolyte level on a regular schedule.

I agree on the zener diode because it is the only diode type that not
only forces the direction of the current (normal diode) but limits
voltage. Flooded battery better but since you already own the AGM, you
could put some type of RESISTIVE material in series. (Large resistor,
light, etc) The key would be to find the right resistance (Approx
2/15th of the internal resistance of the battery) But experimentation
would be reguired

wrote:
On Mon, 12 Nov 2007 12:12:16 -0800, wrote:


I have an older Honda 10 hp outboard that has a charging circuit. The
small boat that it is on has a bilge pump and small AGM battery. The
outboard's charging circuit puts out over 15 volts. Can I put a diode
in the circuit to reduce the voltage? If so what kind of diode?

thanks


A very large zener diode with a substantial heat sink. I doubt you will find one
that can handle the current. Old British motorcycles had them for this purpose,
but that was a LONG time ago, and they were more likely 6 volt systems.

Most small outboards have poor, or no, regulation to speak of. Some
manufacturers have add on regulators, but they are pretty expensive - as in
$200+. It would make more sense in your case, to ditch the AGM and get a flooded
battery that is not so sensitive to voltages over 14. You will need to check the
electrolyte level on a regular schedule.

Is the "more than 15 v " with the battery connected to the generator or when
disconnected and open circuit?
The simplest solution is probably a series resistor to limit the charge to
about 1 amp or less which should not upset a reasonably sized battery.

Try a 21W car bulb and measure the current.

Alec

wrote in message
ups.com...
I have an older Honda 10 hp outboard that has a charging circuit. The
small boat that it is on has a bilge pump and small AGM battery. The
outboard's charging circuit puts out over 15 volts. Can I put a diode
in the circuit to reduce the voltage? If so what kind of diode?

thanks

On Nov 14, 8:01 am, wrote:
On Tue, 13 Nov 2007 19:23:43 -0000, "Alec" wrote:
Is the "more than 15 v " with the battery connected to the generator or when
disconnected and open circuit?
The simplest solution is probably a series resistor to limit the charge to
about 1 amp or less which should not upset a reasonably sized battery.

Try a 21W car bulb and measure the current.

Alec

What? This is gibberish.

Hmmm. Makes perfect sense to me. Maybe you could ask a question about
what you don't understand, and then we can all learn something.

Limiting the charging current to avoid damaging the battery is
certainly a workable approach.

On Nov 12, 3:12 pm, wrote:
I have an older Honda 10 hp outboard that has a charging circuit. The
small boat that it is on has a bilge pump and small AGM battery. The
outboard's charging circuit puts out over 15 volts. Can I put a diode
in the circuit to reduce the voltage? If so what kind of diode?

thanks

You could put a galvanic isolator in line with the alternator
output. This will drop the voltage to about 13.6 and can handle the
current.

Eric

Well, you might be right about the common cable and the cheaper
route/solution being the flooded battery, but I can find a 30 amp
isolator for $140. But if you must have that overpriced Guest stuff,
yeah, you'll pay $300 for it.

BTW, agm's are not quickly ruined by having over 14 volts on them. We
use hundreds of Lifeline agm's where I work and the bulk charge spec.
for them is 14.2-14.4 volts. Float charge should be kept at 13.3 or
under. And once fully charged, can be equalized at 15.5 volts for 8
hours when neccessary. As stated in their documentation: "Regardless
of what some rumors portray, AGM batteries are not as sensitive as
once believed. They often get confused with Gel Batteries, which are
very sensitive to voltage and they need a special charger."

Eric

On Nov 14, 9:06 am, wrote:

Yeah, and a 30 amp galvanic isolator to protect that $150 AGM battery
will run you about $300 or more! If the Honda outboard in question has
electric start, you will need one that will handle 100 amps, as that
is about what the starter motor draws, and the same cables from the
motor to the battery are used for both starting and charging.

I'll repeat: A new flooded battery will solve the issue for about $50.

A series resistor to limit the current to about 1 amp should not cause the
battery voltage to rise too far. The wattage the battery will be expected to
dissipate will only be about 1*14= 14W at this level it should not get warm.

This is not going to overheat or boil a battery of say 75 or more AH. Limit
the current to a lower level if the battery is smaller.

A car bulb makes a good dropping resistor as the resistance rises if the
current rises and they are meant to dissipate power.

This is constant current charging, not constant voltage as from a typical
car alternator. Constant voltage charging can harm any battery if it is too
high as the current will remain at a high level for too long and the battery
will overheat, not good on any sealed battery.

Alec




wrote in message
...
On Wed, 14 Nov 2007 16:05:00 -0800, wrote:


Well, you might be right about the common cable and the cheaper
route/solution being the flooded battery, but I can find a 30 amp
isolator for $140. But if you must have that overpriced Guest stuff,
yeah, you'll pay $300 for it.


Math lesson #1: $140 $50


BTW, agm's are not quickly ruined by having over 14 volts on them. We
use hundreds of Lifeline agm's where I work and the bulk charge spec.
for them is 14.2-14.4 volts. Float charge should be kept at 13.3 or
under. And once fully charged, can be equalized at 15.5 volts for 8
hours when neccessary. As stated in their documentation: "Regardless
of what some rumors portray, AGM batteries are not as sensitive as
once believed. They often get confused with Gel Batteries, which are
very sensitive to voltage and they need a special charger."


Confused? I'm not the one who thinks a $140 work-ariound is superior to a
$50
Solution. :')

And here's a little tip: An AGM battery can be effectively charged at 13.5
volts. A flooded battery needs more than that just to begin charging.
There is a
difference in internal resistance. And for purposes of this thread, the
essentially unregulated voltage from a 10 hp Honda outboard IS enough to
quickly
ruin an AGM battery. That's the subject under discussion. I stated that
AGM's
can be damaged by voltages above 14.

Math lesson #2: 15 volts 14 volts


I'll repeat: A new flooded battery will solve the issue for about $50


Eric

On Nov 14, 9:06 am, wrote:

Yeah, and a 30 amp galvanic isolator to protect that $150 AGM battery
will run you about $300 or more! If the Honda outboard in question has
electric start, you will need one that will handle 100 amps, as that
is about what the starter motor draws, and the same cables from the
motor to the battery are used for both starting and charging.

I'll repeat: A new flooded battery will solve the issue for about $50.

On Nov 15, 6:56 am, wrote:
Confused? I'm not the one who thinks a $140 work-ariound is superior to a $50
Solution. :')

Neither am I. He asked if he could use a diode to drop the voltage
and if so, what kind.


And here's a little tip: An AGM battery can be effectively charged at 13.5
volts.

And your point?

A flooded battery needs more than that just to begin charging.

FALSE!

difference in internal resistance. And for purposes of this thread, the
essentially unregulated voltage from a 10 hp Honda outboard IS enough to quickly
ruin an AGM battery.

Quickly? I doubt it. I doubt the thing can output more than 10 amps if
even that much. Once the AGM is fully charged, it can still stand the
15v for several hours with no ill effects. I have a Lifeline AGM
GPL-24T right here in the shop that's fully charged but been sitting
for a while (no surface charge). I just put a constant 15v on it and
it started charging at 4 amps. After 5 minutes it's down to 2 amps and
in the next few minutes it will be down to a couple hundred milliamps.
I can leave it on this way for hours and it ain't gonna hurt the
battery. I can fully test this battery with my carbon pile load tester
to prove that the battery hasn't been quickly ruined.

Eric

On Nov 15, 12:59 pm, wrote:
Suit yourself. I don't have much faith at this point in your ability
to accurately test and diagnose a battery sitting in front of you. I'm
not even convinced you know the difference between voltage and
current.

And Iv'e only been an electronics technician for 32 years. Gee, I
guess I better go look for another profession.

Eric